1. Field of the Invention
The present invention relates generally to rate control in a mobile communication system.
2. Description of the Related Art
Link Adaption (LA), which is a technology for adapting to a channel change with time in a radio channel environment, includes Closed Loop Rate Control (CLRC) and Open Loop Rate Control (OLRC).
In CLRC, a receiving end estimates quality information of a channel and determines an optimum code rate to obtain a desired target error rate with estimated channel quality. The error rate may be either a Frame Error Rate (FER) or a Packet Error Rate (PER), and the code rate may be a Codec rate. The determined code rate information may be input to a transmitting end such that next data can be transmitted at the corresponding code rate. CLRC has the advantage of adapting quickly to a channel change. However, performance information on channel quality and a code rate should be known in advance. If the advance performance information is different from the performance in a real radio channel environment, optimal adaptation is difficult to achieve.
OLRC is an improvement to overcome the disadvantages of CLRC. That is, when an error rate estimated by a receiving end is higher than a target error rate, a positive (+) offset value is input to a threshold value used in CLRC such that a lower code rate can be selected, and when an error rate estimated by the receiving end is lower than a target error rate, a negative (−) offset value is input to the threshold value such that a higher code rate can be selected. In this manner, OLRC can guarantee a target error rate that is difficult to guarantee with only CLRC.
In OLRC, if the radio channel environment is very good and thus a current error rate becomes better than a target error rate, the offset value decreases, and if the radio channel environment is very poor and thus a current error rate becomes worse than a target error rate, the offset value increases. If the offset value decreases or increases in this manner, there is a problem that it is difficult to adapt quickly when the channel state is reversed. There is a method for limiting a maximum value and a minimum value of the offset in order to solve this problem. However, if the offset is within a narrow range, the benefits of OLRC are not maximized, and if the offset is within a wide range, there is a problem that it is difficult to adapt quickly to a channel change as in the related-art method. Accordingly, there is a need in the art for an improved method for controlling a data rate in a mobile communication system.